1. Field of the Invention
The present invention relates to copper metal powders and to methods for producing such powders, as well as devices incorporating the powders. In particular, the present invention is directed to powder batches of copper metal particles that can have a well controlled average particle size, well controlled particle size distribution spherical morphology and high crystallinity.
2. Description of Related Art
Many product applications require metal-containing powders with one or more of the following properties: high purity; high crystallinity; small average particle size; narrow particle size distribution; spherical particle morphology; controlled surface chemistry; reduced agglomeration; and high density (low porosity). Examples of metal powders requiring such characteristics include, but are not limited to, those useful in microelectronic applications, such as for the internal electrodes and external terminations of multi-layer ceramic capacitors (MLCC's), for conductive traces on hybrid integrated circuits, multilayer ceramics or rnultichip modules, and for resistors and other devices.
Electronic devices such as capacitors, and in particular MLCC's, have traditionally used electrodes fabricated from noble metals such as silver palladium and mixtures/alloys thereof. MLCC's are fabricated by stacking alternate layers of a ceramic dielectric and a conductive metal and then sintering (heating) the stack to densify the layers and obtain a monolithic device. Most ceramic dielectric compounds are oxides that must be sintered at an elevated temperature in an oxygen-containing atmosphere to avoid reduction of the ceramic and the loss of the dielectric properties. Noble metals such as palladium advantageously resist oxidation under these conditions. However, noble metals are relatively expensive and significantly increase the cost of such devices. Therefore, it would be advantageous to utilize less costly base metals for such applications. Base metals such as copper are generally at least an order of magnitude less costly than noble metals. But most base metals tend to oxidize when held in an oxygen-containing atmosphere at elevated sintering temperatures, thereby ruining the electrical properties of the metal and creating other problems in the device, such as delamination of the stacked layers.
Various methods have been disclosed for the production of copper metal powder. U.S. Pat. No. 3,881,914 by Heidelberg discloses a liquid precipitation process for the production of electronic grade copper useful for thick film conductive circuits. Copper sulfate or copper acetate is reacted with hypophosphorous acid to form elemental copper which is separated, washed with an organic and dried in a vacuum. It is disclosed that the copper powders have particle sizes of less than about 1 to 2 μm.
U.S. Pat. No. 4,645,532 by Mackiw et al. discloses a process for the production of copper powder having particles of less than about 5 μm. An ammoniacal cuprous salt solution is acidified in a substantially oxygen-free environment to produce the fine copper powder, which is substantially spherical. It is disclosed that the copper powder has an oxygen content of less than about 1 percent by weight.
Fievet et al. in an article entitled “Controlled Nucleation and Growth of Micrometre-size Copper Particles Prepared by the Polyol Process”, (J. Mater. Chem., Vol. 3, pgs. 627-632, 1993) disclose the preparation of copper particles by dissolving a precursor in a liquid polyol to nucleate and grow copper metal. In one example, the powder had a mean particle size of 0.46 μm with a standard deviation () of 0.26 μm.
Hsu et al. in an article entitled “Preparation and Characterization of Uniform Particles of Pure and Coated Metallic Copper” (Powder Technology, Vol. 63, pgs. 265-275, 1990) disclose a liquid precipitation and reduction process for the production of copper powder. The powder has a narrow size distribution in the 1 to 3 μm size range and the particles are substantially spherical.
U.S. Pat. No. 4,778,517 by Kopatz et al. discloses a process for producing finely divided spherical copper powder. An acidic solution of copper is evaporated, the compounds are milled and than reduced by heating. At least a portion of the compounds are then sprayed into a high temperature zone to melt the particles and form molten droplets. The droplets are cooled to form essentially spherical metal particles of copper or copper alloys having a size of less than about 20 μm.
Other methods have been proposed to produce copper powders. For example, Champion et al. in an article entitled “Preparation and Characterization of Nanocrystalline Copper Powders” (Scripta Materialia, Vol. 35, No. 4, Pages 517-522, 1996) discloses the production of nanocrystalline copper powders having an average particle size of about 35 nanometers with a standard deviation of 16 nanometers. The powders are formed by cryogenic melting, that is, overheating the molten metal in contact with a cryogenic liquid. Significant levels of Cu2O are present on the surface of the particles.
Spray pyrolysis is not commonly used for the production of metal powders, particularly those containing small particles, for example, having an average size of less than about 5 μm. This is believed to be due to the high processing costs and low production rates typically associated with spray pyrolysis. Further, spray pyrolysis methods often produce hollow particles that are not sufficiently densified for most applications.
Nagashima et al. in an article entitled “Preparation of Fine, Spherical Copper Particles by Spray-Pyrolysis Technique” (Nippon Kagaku Kaishi, Vol. 1, Pgs. 17-24, 1990) disclose the preparation of copper particles by a spray pyrolysis technique. Two types of copper particles were observed, those having a spherical morphology and those having an irregular shape. Spherical copper particles were formed when the particles were heated above the melting point of copper for 0.1 seconds or longer. The copper particles were utilized to form copper films having a low resistivity.
In addition to substantially pure copper metal powders, copper metal powders having modifications such as copper alloys, metal-ceramic composites or coated copper powders have been disclosed in the prior art.
For example, U.S. Pat. No. 4,600,604 by Siuta discloses a metal oxide coated copper powder. The copper powder has an average particle size of 1 to 5 μm and the oxide layer is substantially continuous with a thickness of 1 to 20 nanometers. The oxide coating, which is formed from an organometallic coating deposited by solution, controls the sintering and shrinkage characteristics of the particles when used in connection with ceramic substrates.
U.S. Pat. No. 4,781,980 by Yoshitake et al. discloses a coated copper powder for use in a conductive paste composition. An antioxidation film of an organic acid salt is formed on the surface of the copper powder using a liquid route. The coating provides good humidity resistance and thermal resistance to the powder by reducing surface oxidation.
Properties of copper metal powder can also be altered by additives included with the copper metal. For example, U.S. Pat. No. 5,470,373 by Edelstein et al. discloses oxidation resistant copper nanoparticles that include an additive that is phase separated from the copper. The additive can be selected from nickel, cobalt, iron, manganese, cadmium, zinc, tin, magnesium, calcium and chromium. The copper metal powder is produced by a liquid precipitation route.
There remains a need for copper powders having a small particle size, narrow size distribution, high crystallinity (large crystals) and spherical morphology. It would be particularly advantageous if such metal powders could be produced in large quantities on a continuous basis.